The present invention relates to a vacuum circuit breaker having a wipe cage, and more particularly, relates to means for selectively latching the wipe cage.
The usual vacuum circuit breaker comprises a sealed, evacuated envelope containing a stationary contact and a movable contact, with the movable contact being movable from a closed position, in which it abuts the stationary contact to an open position, in which it is separated from the stationary contact. Typically an operating member imparts movement to the movable contact through a coupling member. The operating member separates the contacts during an opening stroke and causes the contacts to abut each other during a closing stroke. The two ends of the wipe means are resiliently biased apart by a spring. The wipe means includes means to limit the maximum distance of separation between the ends. With the movable contact in the closed position, the wipe means ends are separated by less than the maximum distance of separation, whereby latitude in the separation between the movable contact (connected to a first wipe means end) and the operating member (connected to a second wipe means end), or contact wipe, is provided by the wipe means. Such contact wipe accommodates contact wear, provides a predetermined force (that is, the force of the spring) between the contacts when the movable contact is in the closed position, and, before the movable contact travels from the closed position to the open position, the operating member is allowed to build up speed before the wipe means ends are separated to the maximum extent whereby the movable contact is forced to separate abruptly from the stationary contact.
To maintain the interior of the envelope under vacuum, a bellows is typically provided between the coupling member and the envelope. The bellows typically comprises a plurality of folds or convolutions each formed from sheet stainless steel with a typical thickness of three thousandths of an inch. The bellows, thus, is likely to be rather delicate.
Problems can arise during the opening stroke of the operating member if the wipe means spring is subject to contraction owing to severe deceleration of the operating member, which can arise near the end of the opening stroke thereof. If the wipe means spring becomes so contracted, the movable contact, and consequently the bellows, will be subject to "overtravel" or travel beyond that which is necessary for the movable contact to be moved into an open position. Such overtravel subjects the bellows to exacerbated deformation, and is particularly deleterious to the bellows if the convolutions thereof are caused to press againt each other. Additionally, such overtravel is followed by the movable contact rebounding at high speed away from the operating member. Inasmuch as one end of the bellows moves with the movable contact, there will be imposed on the bellows a range of deformation of individual convolutions thereof, or stress, typically, approximately three times the range of deformation of individual convolutions, or stress, imposed on the bellows in the absence of overtravel and high speed rebound. This fact has not been clearly understood before the present invention. Further, in the event that overtravel of the movable contact is extensive enough as to cause the wipe means spring to become fully contracted, the movable contact will impact against the operating member. The consequent motion of the bellows (one end of which moves with the bellows) will produce a particularly high stress therein.
A problem may arise during the closing stroke of the operating member, if, during this time, the wipe means spring has first contracted, owing to severe acceleration of the operating member which can arise near the beginning of the closing stroke thereof, and is then in the process of expanding when the movable contact comes into abutment with the stationary contact. Under these conditions, the velocity of the movable contact, just prior to coming into abutment with the stationary contact, is subject to an increase in value due to the expanding of the wipe means spring. Accordingly, in addition to a large stress generated in the bellows, the movable contact could strike the stationary contact and rebound away from the stationary contact one or more times, until finally coming to rest against the stationary contact. During each rebound, the possibility of severe erosion and welding of the contacts is present.
A known vacuum circuit breaker including the above features further includes a stop which limits the extent to which the wipe means spring can contract, and thus, the maximum attainable contact wipe, to a value such as three-sixteenths inch. The provision of such a stop is useful for limiting overtravel of the movable contact, and thus, the bellows, during the opening stroke of the operating member: however, the remaining overtravel of the movable contact and the bellows is still excessive. Additionally, the high speed rebound of the movable contact and the bellows following overtravel, remains excessive, notwithstanding the provision of the stop. Further, during the closing stroke of the operating member, the problem of the movable contact impacting the stationary contact and rebounding therefrom, with the risk of severe erosion and welding of the contacts, remains substantially unaffected by the provision of the stop.